Reductionops: whd_state_gen can take and answers a cst_stack too

+ cst_stack is kept en a meta/evar is "unfold".

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16142 85f007b7-540e-0410-9357-904b9bb8a0f7